It is known that some micro-organisms that are found in sludge used for aerobic wastewater treatment are able to produce microbial storage compounds, such as for example poly(hydroxyalkanoate) (PHA), as a reserve compound. Recently there is interest in wastewater treatment processes wherein the accumulation of polyhydroxyalkanoate in PHA-accumulating bacteria is maximised in order to recover such polyhydroxyalkanoate from these bacteria, for example for application in bioplastics.
Processes for the selection of micro-organisms capable of producing PHA or other microbial storage compounds are known in the art and typically comprise several cycles alternating a so-called feast phase wherein sludge comprising PHA-accumulating bacteria is fed with a substrate that comprises readily biodegradable organic compounds (so-called readily biodegradable chemical oxygen demand or RBCOD) with a so-called famine phase wherein substrate is withheld from the bacteria. In the feast phase, the PHA-accumulating bacteria are converting RBCOD such as volatile fatty acids into PHA. In the famine phase, the PHA accumulated in the bacteria is used as feed, resulting in selection of those bacteria that are capable of accumulating PHA or other bacterial storage compounds.
For example, WO00/52189 discloses a process for the production of polyhydroxyalkanoate wherein in a first step microorganisms capable of accumulating PHA are selected by alternatingly subjecting the micro-organisms to a phase wherein substrate is added (feast phase) and a phase wherein substrate is withheld (famine phase). In an accumulation step, PHA is accumulated by subjecting some of the micro-organisms selected in the first step to a substrate, preferably a fatty acid-comprising stream. In the process of WO00/52189, the selection step with alternating feast and famine phases is carried out in a first reactor and the accumulation step is carried out in a separate second reactor.
WO2011/073744 also discloses a process for selecting micro-organisms (biomass) capable of producing PHA by alternatingly subjecting biomass to feast and famine conditions. In the process of WO2011/073744, the feast and famine conditions may be applied sequentially in the same reactor, or in separate reactors. Sludge (biomass) withdrawn from the selection process may be subjected to further accumulation in a separate step or to extraction of PHA.
WO2012/023114 discloses a similar process as WO2011073744 for increasing the PHA-accumulating potential of biomass used in the treatment of wastewater by subjecting the biomass to repeated cycles of feast-famine conditions in a Sequencing Batch Reactor (SBR) system, using specific Readily Biodegradable Chemical Oxygen Demand (RBCOD) feeding rates at feast conditions. WO2014/108864, published Jul. 17, 2014, again describes a similar method including a prolonged secondary famine process. In all these methods, any accumulation step is performed in a separate reactor.
WO2013/022874 discloses a process for PHA production using return sludge in an aeration basin in such a manner that a famine-feast regime is provide in the same aeration basin. The return sludge may be treated in a return stream reactor in which a relatively low food-to-biomass ratio is applied. No accumulation is disclosed or suggested.
Serafim et al. Appl. Microbiol. Biotechnol. 81 (2008) 615-628 and Bengtsson et al. Bioresource Technology 99 (2008) 509-516, disclose processes for the production of PHA using mixed cultures in sequencing batch or continuous reactors, wherein the feast and famine cycles may be performed in a single (selection) reactor. However, the accumulation cycle is always performed in a separate reactor.